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Figure 3 Phase 2 reentry-initiated circus movement tachycardia abolished with transient outward current inhibition. (A) Exposure of canine right ventricular epicardial sheet to simulated ischemia results in loss of the dome at sites 3 and 4 but not at sites 1 and 2 (basic cycle length = 1100 ms). Conduction of the basic beat proceeds normally from the stimulation site (site 2; see schematic a). Propagation of the action potential dome from the right half of the preparation causes reexcitation of the left half via a phase 2 reentry mechanism (see schematic b). The extrasystolic beat generated by phase 2 reentry then initiates a run of tachycardia that is sustained for four additional cycles via a typical (phase 0) circus movement reentry mechanism. The proposed reentrant path is shown in Schematic c. Note that phase 2 reentry provides an activation front roughly perpendicular to that of the basic beat. (B) Recorded 5 min after addition of 1 mmol/L 4-aminopyridine (4-AP), an inhibitor of the transient outward current. In the continued presence of ischemia, 4-AP restores the dome at all epicardial recording sites within 3 min. Thus electrical homogeneity is restored and all reentrant activity abolished. Adapted with permission from Lukas and Antzelevitch (92).